Early in the productive lifespan of an oil well, the reservoir is under sufficient pressure to bring the oil to the surface. However, as oil is depleted, at some point the pressure will not suffice to bring oil to the surface and at this point, the natural lift mechanisms must be augmented. Frequently, a downhole pump, for instance an electric submersible pump (ESP), is placed downhole and used to provide artificial lift. However, such pumps are usually designed to pump fluids, with limited vapors, and do not function effectively when vapor enters the pump.
Low-pressure areas are created in the ESP pump stage vanes as they rotate. Gas can build up in this region and reduce the pump efficiency, or even block the passage of the entire vane or multiple vanes in the pump, which may lead to the need to shut down and restart the system, and/or to premature or catastrophic pump failure. This is called “gas-lock” in the industry. However, it is common that “gas-locking” is loosely used by the industry to refer to any and all pump problems triggered by vapors.
It was originally believed that gas-locking or vapor interference would not present significant problems in SAGD operations because the original hydrocarbon gas saturation is very low in these bitumen oil fields. However, the problem of vapor-interference is increasingly becoming recognized as an impediment in SAGD operations, where vapor can enter the system from a number of different sources, some of which are not understood and may not be monitored.
In SAGD, well pairs are typically vertically displaced 4-5 meters from one another, the upper well being used for steam injection to mobilize the heavy oil which then drains to the lower well for production. Although a subcool is maintained at about 7-10° C. in order to ensure that steam does not breakthrough to the producer well, vapor breakthrough can happen, leading to vapor entering the downhole pump.
Hydrocarbon gas from reservoir fluid may break out from solution and become a free gas when it enters into wellbore due to lower pressures there. Additionally, if oil reservoir pressure is lower than saturation pressure, some free gas may exist in reservoir and could flow into wellbore together with reservoir fluid. In the case of hot steam flooding, hot water that enters into wellbore can flash into steam vapor due to lower pressure. Another source of vapor comes from aquathermolysis, which is the reaction involving hot water or steam and reservoir mineral clays and crude hydrocarbon and generates gases like CO2 and H2S. It is important to understand that the volume of these vapors is proportional to the amount of liquid flowing into wellbore and being produced.
Up to now, attempts to address the gas-locking problem have been directed at modifying the downhole pump alone to either minimize gas entry and/or to shuttle it safely aside at some point. Work is also underway to develop multiphase downhole pumps that can pump both liquids and gases. However, none of those efforts solve the problem for existing downhole pumping systems especially for hot SAGD producers.
Some SAGD producers have experienced No Flow or Low Flow (NF/LF), event, sometimes referred as “deadhead”. When NF/LF happens, liquid production gets lost or goes very low. ESPs run with lower amperage but frequency remains constant. As a consequence, NF/LF events cause production down time and become an optimization constraint. A temporary measure to deal with the NF/LF is to quickly drop and resume the pump hertz and although capble of bringing the well back to production, the well cannot be optimized to its productivity potential after such an event.
Thus, there remains a need in the art to solve the vapor interference problem, particularly as relates to SAGD and other steam based heavy oil production methods, and particularly to NF/LF events.